1. Field of the Invention
The present invention relates to a controller for use with a radiocommunications system which controls a plurality of base stations connected to mobile telephones via a radio channel within a business establishment, and more particularly, to a control operation for the purpose of allocating control slots to the base stations, respectively.
2. Description of the Related Art
FIG. 6 is a block diagram showing an existing radiocommunications system used in a business establishment, wherein a PBX (private branch exchange) main unit 1 is connected to a plurality of base stations 2 and to a personal computer PC 3 used for customizing the PBX 1.
The PBX main unit 1 comprises a PBX control section 11 for controlling the overall PBX main unit 1, a control slot frequency management section 12 which manages the frequency of a control slot assigned to each of the base stations 2, a control slot period management section 13 for managing a period of the control slot during which a radio frequency signal is transmitted from the base station 2, a base station interface 14 which serves as an interface between the PBX main unit 1 and the base stations 2, a PC interface 15 which serves as an interface between the PBX unit 1 and the PC 3, and a broadcast call area number management section 16 which manages a broadcast call area number registered for the purpose of preventing the positions of mobile stations from being registered in the adjacent base stations 2 in an overlapping manner.
Each of the base stations 2 comprises a PBX interface 21 which serves as an interface between the PBX main unit 1 and the base station 1, a base station control section 22 for controlling the entire base station 2, a control slot management section 23 which stores the frequency and period of the control slot assigned to each of the base stations 2, and a radio section 24 which emits a radio frequency signal.
At the time of designing of the base stations, with regard to the adjacent base stations 2, the PC 3 sets an identical broadcast call area number in the broadcast call area number management sections 16 by way of the PC interface 15. In a case where a mobile station is handed over from one zone to another zone of the base stations 2 assigned the same broadcast call area number, the position of the mobile station is not registered in the base station.
The PBX control section 11 allocates control slots to the base stations 2 in numerical order of ports of the PBX main unit 1 and thus-allocated control slots are stored in the control slot frequency management section 12 and the control slot period management section 13. Details of allocation of the control slots are stored in the control slot management section 23 of each base station 2 by way of the base station interface 14 and the PBX interface 21. Depending on the settings of the control slot management section 23, each base station 2 determines the frequency and period of the control slot during which a radio frequency signal is transmitted from the base station 2.
FIG. 7 is a table showing broadcast call area numbers respectively assigned to base stations CS1 to CS8, frequencies (i.e., the details of the control slot frequency management section 12) f1 and f2, periods (i.e., details of the control slot period management section 13) "a"-"d" of the respective control slots, and control slots A through H.
FIG. 8 is a schematic representation showing the control slots A to H allocated respectively to the four periods "a" to "d" during one cycle of control channels of two frequencies f1 and f2.
FIG. 7 shows base stations CS1 and CS2 assigned the same broadcast call area number, i.e., 1, and base stations CS5 and CS6 assigned the same broadcast call area number, i.e., 4. The base stations assigned the same broadcast area number are positioned in close proximity to each other. Other base stations CS3, CS4, CS7, and CS8 are not positioned in close proximity to one another, and hence they are respectively assigned individual broadcast call area numbers 2, 3, 5, and 6.
The base stations CS1 to CS4 are allocated a control channel of frequency f1, and the base stations CS5 to CS8 are allocated a control channel of frequency f2. The base stations CS1 to CS4 are respectively sequentially assigned the four periods "a" to "d" into which one cycle of the control channel of frequency f1 is divided. Similarly, the base stations CS5 to CS8 are sequentially and respectively assigned the four periods "a" to "d" into which one cycle of the control signal of frequency f2 is divided. Consequently, as shown in FIG. 8, the base stations CS1 to CS4 are respectively allocated the control slots A to D, and the base stations CS5 to CS8 are respectively allocated the control slots E to H.
If existing radiocommunications systems are provided in close proximity to each other, and if control slots which one of the systems attempts to use have already been used by the other system, it becomes impossible for the former system to control mobile stations existing in a service area covered by base stations of the former system.
FIG. 9 shows a schematic layout showing an example of the adjacent radiocommunications systems and their base stations. Base stations CS1, CS2, and CS3 are connected to one PBX main unit 1A. Base stations CSa and CSb are connected to another PBX main unit B. In this example, the base stations CS1, CS2, and CSa are positioned in close proximity to one another. In this layout, the rate of connection between the mobile station and the base station CS1 or CS2 is decreased by interference from the base station CSa.